1. Field of the Invention
The present invention relates to a multi-beam scanner having a plurality of light emitting points, and to an image forming apparatus, such as a laser-beam printer, copier, and a facsimile machine, incorporated with the multi-beam scanner.
2. Description of Related Art
A conventional multi-beam scanner is known to have a light source unit with a laser diode array that includes a plurality of light sources. The multi-beam scanner uses the light source unit to simultaneously scan a plurality of light beams on the scan surface of a photosensitive body, in order to form an image on the scan surface. Such a multi-beam scanner increases the scanning speed and recording speed of an image forming apparatus.
The multi-beam scanner with the plurality of light sources has to produce, on the scan surface, the plurality of laser beam spots with the same, desired diameter, similarly to a scanner having a single light source. If the plurality of laser beams produce different spot diameters on the scan surface, then pixels will also become different sizes when recorded. As a result, images will become deformed or curved, or light and shade portions will be developed on the image, so that high quality images cannot be obtained.
Japanese Patent-Application Publication No. 62-161117 discloses a scanner which is provided with a mechanism for adjusting the position of the light source in a focus depth direction. In this scanner, the position of the laser array is adjusted in the focus depth direction using a screw and is fixed using a spring. As a result, the position of a collimator lens and the plurality of light sources can be adjusted so that the laser beams emitted from all the light sources have the same spot diameter along the focal depth direction.
However, this scanner requires a great number of components, which makes the scanner expensive. Further, because the position of the laser array is adjusted and fixed in the manner described above, the laser array will possibly be displaced out of position when the scanner is bumped. When the position is displaced, the desired spot diameter cannot be obtained on the scan surface.
Japanese Patent-Application Publication No. 9-26550 discloses a multi-beam scanner that reduces variations in the spot diameter that are caused by variations in image positions of the different light beams in the depth direction of the light beams. According to this multi-beam scanner, the position where the image should be formed in terms of design (desired spot position) exists in dispersion in the depth direction of the image formation positions (spot positions) of plural light beams, so that variation in spot diameter can be reduced to a minimum.
However, in the multi-beam scanner disclosed in Japanese Patent-Application Publication No. 9-26550, the optical system has a certain amount of curvature of field so that as the scanning position changes, the focal points of the laser beams will move in forward and backward (depth) directions. Therefore, depending on the scanning position, the focal points may not be properly located on the scan surface, so that printing quality suffers.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved multi-beam scanner having a plurality of light emitting points that are all properly focused on an object to be scanned at all the scanning positions, regardless of the curvature of field of the optical system employed therein.
In order to attain the above and other objects, the present invention provides a multi-beam scanner comprising: a light-emitting unit which has a plurality of light emitting portions for emitting a plurality of light beams, the plurality of light emitting portions being separated from one another in a depth direction of the light beams; a scanning unit that deflects the light beams in a main scanning direction in a plurality of lines across a surface of an object to be scanned, the plurality of lines being arranged along an auxiliary scanning direction that is substantially perpendicular to the main scanning direction; and a light converging unit converging the plurality of light beams onto the surface of the object to be scanned, the light converging unit allowing depths of focus of the light beams from all the light emitting portions to overlap on the surface of the object.
Thus, according to the present invention, the multi-beam scanner includes: a light-emitting unit that emits light beams from a plurality of light emitting points, a scanning unit that deflects the light beams to scan the light beams on an object to be irradiated, and light converging unit which is disposed between the light-emitting unit and the object to be irradiated. An optical system is constructed from the light-emitting unit, the scanning unit, and the light converging unit.
The light-emitting unit has light emitting points that are shifted from one another in the depth direction of the light beams. The optical system has a characteristic value, such as a longitudinal magnification or an image side numerical aperture, which is set to a value so that the depths of focus of light beams from all the light emitting points overlap at least partly with one another along the depth direction, and the object to be irradiated is located within the range where the depths of focus overlap.
With this configuration, because the light emitting points are separated by a distance in the depth direction, the depths of focus in the light beams from the respective light emitting points are shifted from one another at each scanning position on the object to be irradiated. To insure that the light beams from all the light emitting points be properly focused on the object at all the scanning positions, the scan surface of the object to be scanned has to be located within the range where the depths of focus of all the light beams overlap.
In order for the depths of focus of all the beams to overlap, the value derived by subtracting the shift (xcex94S) in the depths of focus from the range (de) of the depths of focus has to be positive. In other words, it is necessary to satisfy the following inequality:
dexe2x88x92xcex94S greater than 0.
It is noted that the shift amount (xcex94S) can be obtained by multiplying the longitudinal magnification xcex1 of the optical system by the distance xcex94Z that separates the light emitting points from one another in the depth direction. That is, xcex94S=xcex1xc2x7xcex94Z. Therefore, by setting the longitudinal magnification a to an appropriate value, the value, which is derived by subtracting the shift (xcex94S) from the range (de), will become positive, so that the depths of focus in all the light beams will overlap.
By positioning the surface of the object to be scanned within the range where the depths of focus overlap, the light beams from all the light emitting points will be properly focused on the object surface.
Especially, according to the multi-beam scanner of the present invention, the longitudinal magnification xcex1 of the optical system is preferably set in association with depth of focus de according to the following inequality:
dexe2x88x92(xcex1xc2x7xcex94Z+Cf) greater than 0xe2x80x83xe2x80x83(A)
wherein
xcex94Z is the distance separating the light emitting points in the depth direction; and
Cf is the amount of the curvature of field caused by the optical system.
By setting the longitudinal magnification xcex1 to satisfy the inequality (A) described above, it can be ensured that the light beams from the all light emitting points will be properly focused on the surface of the object to be scanned, regardless of the scanning position and regardless of the amount of the curvature of field.
It is noted that according to the multi-beam scanner of the present invention, even when the light emitting surface of the light-emitting unit is not located on a plane normal to the optical axis, the optical system can be easily designed by satisfying the above-described relationship (A) while substituting the value of (xcex94z cos xcfx86+xcex94p sin xcfx86) for the distance xcex94Z in the above-described inequality (A), wherein xcfx86 is an angle defined between the light emitting surface of the light-emitting unit and the plane normal to the optical axis of the light converging unit, xcex94p is a distance separating the plurality of light emitting points along a plane parallel to the light emitting surface, and xcex94Z in (xcex94z cos xcfx86xcex94p sin xcfx86) is the distance separating the light emitting points along a direction normal to the light emitting surface.
Because the light-emitting unit is extremely small, it is difficult to position the light-emitting unit so that the light emitting surface is precisely normal to the optical axis. However, by designing the optical system using the inequality that takes into consideration the orientation of the light emitting surface, images with high quality can be reliably obtained.
It should be noted that the optical system may be designed so that the depths of focus in the light beams, only in the main scanning direction, will overlap at least partly with one another. This is because generally the depth of focus, in the auxiliary scanning direction, is longer than that in the main scanning direction. Therefore, by designing the optical system so that the depths of focus in the main scanning direction overlap, the depths of focus in the auxiliary scanning direction will also always overlap. Thus designing the optical system is simple because attention need only be paid to the main scanning direction. Also, even though the optical system is easy to design, the optical system can produce high quality images.
Accordingly, when considering the main scanning direction, the same relationship, as indicated in inequality (A), should be satisfied.
More specifically, the longitudinal magnification xcex1m, along the main scanning direction, of the optical system should be determined by the following inequality (B) in association with the focal depth dem along the main scanning direction:
demxe2x88x92(xcex1mxc2x7xcex94Zm+Cfm) greater than 0xe2x80x83xe2x80x83(B)
wherein
xcex94Zm is a distance separating, in the depth direction, the central positions of the radiations, along the main scanning direction, from the light emitting portions; and
Cfm is the amount of the curvature of field, along the main scanning direction, in the optical system.
When the inequality (B) is satisfied, the effects the same as those described above can be obtained.
It is noted that the light emitting surface of the light-emitting unit may not be oriented completely normal to the optical axis of the optical system. In this case, the amount of (xcex94zm cos xcfx86+xcex94p sin xcfx86) may be substituted for the distance (xcex94Zm) in the inequality (B), wherein xcfx86 is an angle defined between a light emitting surface of the light-emitting unit and a plane normal to the optical axis of the light converging unit; xcex94p is a distance, along a plane parallel to the light emitting surface, separating the plurality of light emitting portions; and xcex94zm in (xcex94zm cos xcfx86+xcex94p sin xcfx86) is a distance, along a direction normal to the light emitting surface, separating the central positions of radiations, along the main scanning direction, from the plurality of light emitting portions.
Thus, according to the present invention, the light beams from all the light emitting points will be reliably focused onto the surface of the object to be scanned.
The longitudinal magnification xcex1m, along the main scanning direction, of the optical system can be set in a manner described below.
For example, the light converging unit may include: a collimate lens for collimating, into approximate parallel beams, the light beams emitted from the light emitting points, and a scan lens (fxcex8 lens) for scanning the collimated light beams on the surface of the object to perform a main scanning operation. In this case, the focal length fco of the collimate lens and the focal length fm, in the main scanning direction, of the scan lens should satisfy the following inequality (C):
(fm/fco)2xc2x7xcex94zm+Cfm less than demxe2x80x83xe2x80x83(C).
The inequality (C) can be obtained from the inequality (B) as described below.
The longitudinal magnification xcex1m, in the main scanning direction, can be represented using the following equation:
xcex1m=(fm/fco)2,
wherein
fm is the focal length in the main scanning direction of the scan lens; and
fco is the focal length of the collimator lens.
By applying the above-described equation to the inequality (B), the inequality (C) can be obtained.
Thus, as described above, high quality images can be obtained by designing the collimate lens and the scan lens to satisfy the inequality (C).
It is noted that if the focal depth de in the inequality (A) is taken into consideration, it can be said that the optical system should be designed to satisfy the following inequality (D):
de greater than (xcex1xc2x7xcex94Z+Cf)xe2x80x83xe2x80x83(D).
It is noted that the focal depth de is closely related to the image-side numerical aperture NA of the optical system.
The inequality (D) can be expressed by the following inequality (E):
2(xcex/2NA2+2y/NAxe2x88x922W0/NA2) greater than xcex1xc2x7xcex94Z+Cfxe2x80x83xe2x80x83(E)
wherein
xcex94Z is a distance separating the light emitting points in the depth direction,
xcex1 is a longitudinal magnification of the optical system,
Cf is the amount of the curvature of field in the optical system;
W0 is an inherent wavefront aberration of the optical system;
xcex is a wavelength of the light beams; and
y is a height of an image.
Thus, the distance xcex94Z can be determined in association with the image-side numerical aperture NA.
As the amount of the image-side numerical aperture NA decreases, the amount of the distance xcex94Z separating the focal depths of the light beams will increase. As the amount of the image-side numerical aperture NA increases, the amount of the distance xcex94Z will decrease. Accordingly, by setting the image-side numerical aperture NA to a proper value, light beams from all the light emitting points can reliably be focused onto the surface of the object regardless of the scanning position and of the amount of curvature of field.
Even when the light emitting surface of the light-emitting unit is not oriented completely normal to the optical axis of the optical system, by substituting the amount of (xcex94z cos xcfx86+xcex94p sin xcfx86) for the distance (xcex94Z) in the inequality (E), the optical system can be easily designed to satisfy the inequality (E), wherein xcfx86 is an angle defined between a light emitting surface of the light-emitting unit and a plane normal to the optical axis of the light converging unit; xcex94p is a distance separating the plurality of light emitting portions along a plane parallel to the light emitting surface; and xcex94z in (xcex94z cos xcfx86+xcex94p sin xcfx86) is a distance separating the plurality of light emitting portions along a direction normal to the light emitting surface.
Because the light-emitting unit is extremely small, it is difficult to locate the light-emitting unit so that the light emitting surface will be completely normal to the optical axis. However, by designing the optical system using the above-described inequality that is modified by taking tilt into consideration, images with high quality can be reliably obtained.
It should be noted that the optical system may be designed considering that the depths of focus of the light beams, only in the main scanning direction, will overlap at least partly with one another. This is because generally the depth of focus in the auxiliary scanning direction is longer than that in the main scanning direction. Therefore, by designing the optical system so that the depths of focus in the main scanning direction overlap, the depths of focus in the auxiliary scanning direction will always overlap, too. Designing the optical system is simple because attention need only be paid to the main scanning direction. Also, even though the optical system is easy to design, the optical system can produce high quality images.
Accordingly, when considering the main scanning direction, the same relationship, as indicated in inequality (E), should be satisfied.
More specifically, the inequality (E) can be represented by the following inequality (F):
2(xcex/2NAm2+2ym/NAmxe2x88x922W0/NAm2) greater than xcex1xc2x7xcex94Zm+Cfmxe2x80x83xe2x80x83(F)
wherein
xcex94Z, is a distance separating, in the depth direction, the center portions of radiations, along the main scanning direction, from the light emitting portions,
xcex1m is a longitudinal magnification, along the main scanning direction, of the optical system,
Cfm is the amount of the curvature of field, along the main scanning direction, in the optical system;
W0 is an inherent wavefront aberration of the optical system;
xcex is a wavelength of the light beams; and
ym is a height of an image.
Accordingly, the distance xcex94Zm can be determined by the inequality (F) in association with the image-side numerical aperture NAem along the main scanning direction.
The light emitting surface of the light-emitting unit may not be oriented completely normal to the optical axis of the optical system. In this case, the amount of (xcex94zm cos xcfx86+xcex94p sin xcfx86) may be substituted for the distance (xcex94Zm) in the inequality (F), wherein xcfx86 is an angle defined between a light emitting surface of the light-emitting unit and a plane normal to the optical axis of the light converging unit; xcex94p is a distance, along a plane parallel to the light emitting surface, separating the plurality of light emitting portions; and xcex94zm in (xcex94zm cos xcfx86+xcex94p sin xcfx86) is a distance, along a plane normal to the light emitting surface, separating the central positions of radiations, along the main scanning direction, from the plurality of light emitting portions.
According to another aspect, the present invention provides an image forming device comprising: a photosensitive body driven to rotate in the auxiliary scanning direction; and a multi-beam scanner, which includes: a light-emitting unit which has a plurality of light emitting portions for emitting a plurality of light beams, the plurality of light emitting portions being separated from one another in a depth direction of the light beams; a scanning unit that deflects the light beams in a main scanning direction in a plurality of lines across a surface of the photosensitive body, the plurality of lines being arranged along an auxiliary scanning direction that is substantially perpendicular to the main scanning direction; and a light converging unit converging the plurality of light beams onto the surface of the photosensitive body, the light converging unit allowing depths of focus of the light beams from all the light emitting portions to overlap on the surface of the photosensitive body, thereby serially irradiating the plurality of light beams on the photosensitive body to form latent images.
By thus applying the multi-beam scanner of the present invention to the image forming device, such as a laser beam printer, a facsimile machine, a copy machine, or the like, that includes the photosensitive body, the image forming device can produce images with high quality.
Further, by designing the image forming device so that an intermediate point, in the range where the depths of focus of all the beams overlap, falls on the surface of the photosensitive drum, the light beams will reliably be focused on the surface of the photosensitive body, even when the collimate lens, the scanning lens, or other components of the optical system are attached at wrong positions or when the refracting power of the optical system changes due to fluctuations in the ambient environment.